1. Technical Field
This invention relates to a process cartridge and an image forming apparatus and method, and in particular, to a system employed in the image forming apparatus with an electrophotographic system to stabilize image quality. Especially, this invention relates to a system capable of suppressing generation of a defective image such as a ghost image, etc., generally caused by a residual voltage remaining on a latent image bearer.
2. Related Art
In an electrophotographic system, an electrostatic latent image is initially formed on a photoreceptor as a latent image bearer electrically charged uniformly beforehand. The electrostatic latent image is subsequently rendered visible as a toner image using toner in a development process. The toner image is subsequently transferred onto a transfer medium, such as a recording sheet, etc., and is subsequently fixed thereon. The transfer medium with the printed image is then outputted from the electrophotographic system.
As devices to execute the above-described series of processes, an electrostatic charging unit, a latent image writing unit, a developing unit, a transfer device, and a fixing device are typically used, respectively.
As the electrostatic charging unit, either a non-contact type electrostatic charging system or a contact type electrostatic charging system may be employed. As a non-contact type electrostatic charging system, a corona discharging system is well known. As a contact type electrostatic charging unit there is a system that employs a proximity electrostatic charging unit to place the electrostatic charging unit near the latent image bearer with a given amount of clearance. Also known as a contact type electrostatic charging system is a system that brings an electrostatic charging unit, such as a charging brush, a charging roller, etc., in contact with a latent image bearer.
The developing unit utilizes one-component developer or two-component developer, As the one-component developer, only developer such as magnetic toner, etc., capable of standing the particles up on end by itself is used, for example. As the two-component developer, toner particles and carriers, such as iron filings, etc., capable of raising the particles on end while bearing the toner particles on the carriers are used.
In a conventional copier, printer, or multifunctional machine, since high-speed performance, high image reproducibility, long-term stability of image quality, quick startup performance, stability of electrostatic charging of toner, etc., are required, a two-component developing unit using the two-component developer is frequently adopted. By contrast, in a compact printer or facsimile machine expected to save cost and space or the like, a one-component developing unit with the one-component developer is frequently adopted. Today, since an output image is increasingly colorized, a stabilized high quality image is more intensively demanded than before.
Typically, a latent image is formed as a latent image pattern on a photoreceptor acting as a latent image bearer by a latent image forming device such as an optical writing unit, etc., using a laser light beam after or at the same time as the electrophotographic photoreceptor is electrically charged uniformly in accordance with an image pattern to be obtained.
To consistently obtain a quality image, it is of course important to stabilize both latent image formation and visualization such as development, etc., of a latent image. To further stabilize image quality, it becomes important to uniformly maintain an electrical charge on the electrophotographic photoreceptor while suppressing variation in electrostatic charge potential that sometimes appears locally within the site of latent image formation.
The above-described contact and non-contact type systems can be selectively employed in the electrostatic charging unit. However, although it is capable of electrostatically charging a target uniformly with electricity, the non-contact type system discharges a great amount of ozone or similar material when operated. Accordingly, the above-described contact type electrostatic charging system is often used as the electrostatic charging unit to avoid such a problem.
Also, a potential on the electrophotographic photoreceptor needs to be controlled within a given range by detecting its chronological deviation to obtain a quality image, especially with reproducibility in a halftone portion thereof. Consequently, an electrically charged potential on the electrophotographic photoreceptor is typically controlled and used within a range, in which an amount of electrical charge applied to the electrophotographic photoreceptor is proportional to the electrically charged potential thereof.
In this respect, various systems described herein below have been proposed to meet the above-described requirements.
For example, a first conventional system utilizes a ghost image detecting system to detect occurrence of a ghost image in a halftone portion on the electrophotographic photoreceptor by recognizing a difference in at least one of a surface potential and a toner adhering amount on the electrophotographic photoreceptor. In such a system, a condition of forming an image on the electrophotographic photoreceptor is corrected by a prescribed correcting system to check the occurrence of the ghost image based on a pattern formed thereon.
A second conventional system employs a residual voltage detecting unit to detect a residual voltage remaining on a photoreceptor surface between a transfer device and an auxiliary electrostatic charging unit arranged downstream of the transfer device in a rotative direction of the electrophotographic photoreceptor. In this system, a history of a residual image is cancelled by adjusting a condition of a bias voltage applied to the auxiliary electrostatic charging unit in accordance with the residual voltage detected by the residual voltage detecting unit.
A third conventional system employs a pre-electrostatic charging process-biasing device located downstream of a lubricant coating unit and upstream of an electrostatic charging unit to apply a prescribed biasing potential, which is greater than an electrical charge start voltage having the same polarity therewith applied by the electrostatic charging unit, to a section of an electrophotographic photoreceptor prior to an electrostatic charging process. In this system, a history of a residual image generated by a pre-transfer process exposing process is cancelled.
A fourth conventional system simply handles an amount of electrical charge generated in developer without applying an electrostatic charging bias.
Since it is important for the image forming apparatus to evenly obtain prescribed image quality in an image portion, an electrophotographic photoreceptor needs a prescribed amount of electrical charge potential thereon, and various methods of controlling the electrical charge potential have been practically proposed as of today.